Mix the miso and sugar in a pan, warm through gently until the sugar is dissolved, brush half over the mackerel fillets, then pop on a tray and grill for 1 to 2 minutes.
Meanwhile, whisk the flour and water together, dip the onions in and deep fry until crispy, drain on kitchen paper and season with sea salt.
Whisk all the ingredients together for the dressing then spoon over the watercress.
To serve, spoon the remaining sauce onto plates, top with fish and onions, then garnish with dressed watercress.
Mix the miso and sugar in a pan, warm through gently until the sugar is dissolved, brush half over the mackerel fillets, then pop on a tray and grill for 1 to 2 minutes.
Meanwhile, whisk the flour and water together, dip the onions in and deep fry until crispy, drain on kitchen paper and season with sea salt.
Whisk all the ingredients together for the dressing then spoon over the watercress.
To serve, spoon the remaining sauce onto plates, top with fish and onions, then garnish with dressed watercress.